What is an operating system?
Operating systems are the main component of system software of computers, smartphones & tablets.
The best known mobile operating systems are Android and iOS.
Android has many variations, also known as #dic:customrom.
What operating system is on my phone?
The Fairphones come with custom Android roms.
If you didn’t #dic:flash anything else your FP2 is running Fairphone OS (with #dic:gapps).
To get further info which exact version you are running check Settings -> About phone -> Build number.
What operating systems are there for Fairphones?
Find a full list at #oslist.